In air conditioning systems thermal storage devices are employed as thermal energy sinks storing energy in a phase change medium (PCM). One such medium is water and known systems employ the refrigeration of the water to form ice. The circulation of a primary or secondary refrigerant is circulated through heat exchanger elements to form the ice in a storage tank. The apparatus is often powered by electrical energy and the formation of the ice can therefore be performed when electrical charges from the supply utility are minimal. The ice formed in the storage apparatus can subsequently be used for refrigerating in air conditioning or other systems as required.
In prior apparatus such as described in PCT application WO88/09261 a heat exchanger element is immersed in a tank of water. The element is manufactured from extruded polyethylene tubes wound into the form of spiral coils held in place by spacer bars. Individual spirals are connected to inlet and outlet manifolds such that the spirals are in generally planar configuration. The coils are not restrained against vertical motion to allow for expansion and contraction in the freezing and melting of ice formed on the coils. A secondary refrigerant such as a brine solution is circulated through the heat exchanger coils at a suitably low temperature to induce ice formation.
In another prior art apparatus heat exchanger coils of galvanised steel are arranged in a vertical plane to form a heat exchanger unit. Several units filled with water as the PCM are arranged in spaced parallel planes in a tank to form a thermal storage system. The coils in a given unit form a serpentine path from the base to the top of the unit. In this type of system experience has shown that as ice builds about the heat exchanger coils a certain amount of cold spills off the vertical ice walls causing extra ice to build at the base of the tank and progressively up the sides of each ice making unit. Over a period of time this buildup can block the flow of water and prevent the system from operating. This occurs as the incoming water is fed in at the bottom of each unit. This system also employs an air pump to produce turbulence in the tank to promote heat exchange between the heat exchanger coils and the storage medium, water. This agitation is further inhibited by the buildup of ice in the lower portions of the tank about the base of each unit.
Another prior art system is described in U.S. Pat. Nos. 4,403,645 and 4,294,078 to MacCracken. A tank is filled with a roll of a flexible tubing mat with rubberized hair spacing the successive layers of the mat. The mat comprises a grid of twin tubings kept closely spaced and fixed to a rigid plastic strip by a flexible plastic strip. Coolant (the heat transfer liquid) is circulated through the tubing. This system involves a complex manufacturing procedure and results in a large volume of the tank being occupied by the mat reducing the available capacity for the PCM employed.
The present invention seeks to overcome disadvantages in these prior art systems or to substantially ameliorate them.